1. Field of the Invention
The present invention relates to a lens unit incorporating, for example, a variable-magnification optical system, and to an image-sensing apparatus incorporating such a lens unit.
2. Description of Related Art
In recent years, digital cameras, which use an image-sensing device such as a CCD (charge-coupled device) to convert an optical image into an electrical signal, have been becoming increasingly popular. As with cameras using silver-halide film (silver-halide film cameras), such digital cameras too are required to be compact and have a simple construction.
Many variable-magnification optical systems (such as zoom lenses) for use in digital cameras and the like include four lens groups respectively having a positive, a negative, a positive, and a positive optical power (refractive power) from the object side. In such variable-magnification optical systems, the first lens group, i.e., the most object-side one, is often kept stationary during magnification variation (zooming) or the like, because this helps simplify the construction. An example of such a construction is disclosed in Japanese Patent Application Laid-open No. H08-248318, laid-open on Sep. 27, 1996 (hereinafter Patent Publication 1).
Disadvantageously, keeping the first lens group stationary as in the zoom lens proposed in Patent Publication 1 tends to give a zoom lens an unduly great total length or an unduly large lens (for example, front lens) diameter. This tendency can be overcome by adopting a magnification variation method that requires the first lens group to be moved. This method certainly contributes to giving a zoom lens a satisfactorily small total length and a satisfactorily small lens diameter, but requires a complicate construction.
To overcome this drawback, the zoom lens proposed in Patent Publication 1 attempts to achieve compactness in lens diameter and the like by adopting a magnification variation method that, while keeping the first lens group stationary, lets the fourth lens group, i.e., the most image-side one, move, or by bending the optical axis with a rectangular prism provided within the first lens group.
However, with this magnification variation method alone, i.e., simply by letting the fourth, most image-side lens group move while keeping the first lens group stationary, it is impossible to attain compactness in lens diameter and the like beyond a certain limit.